It can grow to 2 meters in length and weigh up to 96 kilograms (212 lb). It can live for 25 years and usually attains sexual maturity between ages two and four,[4] but cod in the northeast Arctic can take as long as eight years to fully mature.[5] Colouring is brown to green, with spots on the dorsal side, shading to silver ventrally. A lateral line is clearly visible. Its habitat ranges from the shoreline down to the continental shelf.

Several cod stocks collapsed in the 1990s (declined by >95% of maximum historical biomass) and have failed to recover even with the cessation of fishing.[6] This absence of the apex predator has led to a trophic cascade in many areas.[6] Many other cod stocks remain at risk. The "Atlantic cod" is labelled VU (vulnerable) on the IUCN Red List of Threatened Species.[7]

Adult cod form spawning aggregations from late winter to spring.[8][9] Females release their eggs in batches,[10] and males compete to fertilize them.[11][12][13] Fertilized eggs drift with ocean currents and develop into larvae. Age of maturation varies between cod stocks, from ages two to four in the west Atlantic,[14] but as late as eight years in the northeast Arctic.[5] Cod can live for 13 years or more.[15]

Atlantic cod are a shoaling species and move in large size-structured aggregations led by larger scouts, particularly during post-spawningmigrations inshore for feeding. Cod actively feed during migration and changes in shoal structure occur when food is encountered. Shoals are generally thought to be relatively leaderless and equal in status and distribution of resources and benefits.[16] However, some studies suggest that leading fish gain certain feeding benefits. One study of a migrating Atlantic cod shoal showed significant variability in feeding habits based on size and position in the shoal. Larger scouts consumed a more variable, higher quantity of food while trailing fish had less variable diets and consumed less food. Fish distribution throughout the shoal seems to be dictated by fish size and it has been hypothesized that ultimately, the smaller lagging fish benefit from shoaling because they are more successful in feeding in the shoal than they would be if migrating individually because of social facilitation.[17]

Atlantic cod are apex predators and adults are generally free from the concerns of predation.[18] Juvenile cod, however, may serve as prey for adult cod, which sometimes practice cannibalism. Research has revealed that juvenile cod make substrate decisions based on risk of predation. Without apparent risk of predation, juvenile cod demonstrated a preference for finer-grained substrates like sand and gravel-pebble. However, in the presence of a predator, they preferred to seek safety in the space available in interstitial spaces of a cobble substrate. Selection of cobble significantly reduces the risk of predation. Without access to cobble, the juvenile cod simply tries to escape a predator by fleeing.

Additionally, juvenile Atlantic cod vary their behavior according to the foraging behavior of predators. In the vicinity of a passive predator, cod behavior changes very little. The juveniles prefer finer grained substrates and otherwise avoid the safer kelp, steering clear of the predator. In contrast, in the presence of an actively foraging predator, juveniles are highly avoidant and hide in cobble or in kelp if cobble is unavailable.[19]

As apex predators, heavy fishing of cod in the 1990s and the collapse of American and Canadian cod stocks resulted in trophic cascades. Population limits of several species including American lobsters, crabs, and shrimp have decreased significantly, and the abundance of these species and their increasing range serve as evidence of the Atlantic cod’s role as a major predator rather than prey.[18]

Atlantic cod have been recorded to swim at speeds of a minimum of 2–5 cm/s and a maximum of 21–54 cm/s with a mean swimming speed of 9–17 cm/s. In one hour, cod have been recorded to cover a mean range of 99 to 226 square meters. Swimming speed was higher during the day than at night. This is reflected in the fact that cod more actively search for food during the day. Cod likely modify their activity pattern according to the length of daylight and thus activity will very with time of year.[20]

Swimming and physiological behaviors change in response to fluctuations in water temperature. Respirometry experiments show that heart rate of Atlantic cod changes drastically with changes in temperature of only a few degrees. A rise in water temperature causes marked increase in cod swimming activity. Cod typically avoid new temperature conditions, and the temperatures can dictate where they are distributed in water. They prefer to be deeper, in colder water layers during the day, and in shallower warmer water layers at night. It has been suggested that these fine-tuned behavioral changes to water temperature are driven by an effort to maintain a physiological status quo in order to preserve energy. This is demonstrated by the fact that a change of only 2.5°C caused a highly costly increase in metabolic rate of 15 to 30%.[21]

Stomach sampling studies have discovered that Atlantic cod feed primarily on Crustacea for small cod, and fish for larger ones.[22] In certain regions, the main food source is decapods with fish as a complementary food item in the diet.[23] Wild Atlantic cod throughout the North Sea depend, to a large extent, on commercial fish species also used in fisheries, such as mackerel, haddock, whiting, herring, plaice, and sole, making fishery manipulation of cod significantly easier.[22] Ultimately, food selection by cod is affected by the food item size relative to their own size. However, providing for size, cod do exhibit food preference and are not simply driven by availability.[22]

Atlantic cod practice some cannibalism. In the southern North Sea, 1-2% (by weight) of stomach contents for cod larger than 10 cm consisted to juvenile cod. In the northern North Sea, cannibalism was higher, at 10%.[22] Other reports of cannibalism have estimated as high as 56% of the diet consists of juvenile cod.[24]

Atlantic cod reproduce during 1-2 month spawning seasons annually. Males and females aggregate in spawning schools and each spawning season yields an average of 8.3 egg batches. Females release gametes in a ventral mount, and males then fertilize the released eggs. Evidence has been discovered to suggest that male sound production and other sexually selected characteristics allow female cod to actively choose a spawning partner. Males also exhibit aggressive interactions for access to females.[25] Based on behavioral observations of cod, some researchers have hypothesized that cod mating systems resemble those of lekking species, which is characterized by males aggregating and establishing dominance hierarchies, at which point females may visit and choose a spawning partner based on status and sexual characteristics.[24]

Cod males experience reproductive hierarchies based on size. Larger cod males are ultimately more successful in mating and produce the largest proportion of offspring in a population. However, cod males do experience high levels of sperm competition. In 75% of examined spawning in one study, sperm from multiple males contributed to offspring. As a result of high competition and unpredictable paternity, males may engage in varied mating strategies and may invest in courtship or may simply ejaculate with other spawning couples. Spawning success also varies according to male size relative to female size. Males that are significantly smaller than females demonstrate significantly lower success rates relative to males that are larger than females.[26]

Atlantic cod act as intermediate, paratenic or definitive hosts to a large number of parasite species: 107 taxa listed by Hemmingsen and MacKenzie (2001)[27] and seven new records by Perdiguero-Alonso et al. (2008).[27] The predominant groups of cod parasites in the northeast Atlantic were trematodes (19 species) and nematodes (13 species), including larval anisakids, which comprised 58.2% of the total number of individuals.[27] Parasites of Atlantic cod include copepods, digeneans, monogeneans, acanthocephalans, cestodes, nematodes, myxozoans and protozoans:[27]

The northwest Atlantic cod has been regarded as heavily overfished throughout its range, resulting in a crash in the fishery in the United States and Canada during the early 1990s.

Newfoundland's northern cod fishery can be traced back to the 16th century. On average, about 300,000 tonnes of cod were landed annually until the 1960s, when advances in technology enabled factory trawlers to take larger catches. By 1968, landings for the fish peaked at 800,000 tonnes before a gradual decline set in. With the reopening of the limited cod fisheries in 2006, nearly 2,700 tonnes of cod were hauled in. In 2007, offshore cod stocks were estimated at one per cent of what they were in 1977.[29]

Technologies that contributed to the collapse of Atlantic cod include engine-powered vessels and frozen food compartments aboard ships. Engine-powered vessels had larger nets, greater range, and better navigation. The capacity to catch fish became limitless. In addition, sonar technology gave an edge to catching and detecting fish. Sonar was originally developed during WWII to locate enemy submarines, but was later applied to locating schools of fish. These new technologies, as well as bottom trawlers that destroyed entire ecosystems, contributed to the collapse of Atlantic cod. They were vastly different from old techniques used, such as hand lines and long lines.[30]

The fishery has yet to recover, and may not recover at all because of a possibly stable change in the food chain. Atlantic cod was a top-tier predator, along with haddock, flounder and hake, feeding upon smaller prey, such as herring, capelin, shrimp and snow crab.[6] With the large predatory fish removed, their prey have had population explosions and have become the top predators, affecting the survival rates of cod eggs and fry.

In the winter of 2011-2012, the cod fishery succeeded in convincing NOAA to postpone for one year the planned 82% reduction in catch limits. Instead the limit will be reduced by 22%. The fishery brought in $15.8 million in 2010, coming second behind Georges Bank haddock among the region’s 20 regulated bottom-dwelling groundfish. Data released in 2011 indicated that even closing the fishery would not allow populations to rebound by 2014 to levels required under federal law. Restrictions on cod effectively limit fishing on other groundfish species with which the cod swim, such as flounder and haddock.[32]

Cod populations or stocks can differ significantly both in appearance and biology. For instance, the cod stocks of the Baltic Sea are adapted to low-salinity water. Organisations such as the Northwest Atlantic Fishery Organization (NAFO) and ICES divide the cod into management units or stocks; however, these units are not always biologically distinguishable stocks. Some major stocks/management units on the Canadian/US shelf (see map of NAFO areas) are the Southern Labrador-Eastern Newfoundland stock (NAFO divisions 2J3KL), the Northern Gulf of St. Lawrence stock (NAFO divisions 3Pn4RS), the Northern Scotian Shelf stock (NAFO divisions 4VsW), which all lie in Canadian waters, and the Georges Bank and Gulf of Maine stocks in United States waters. In the European Atlantic, the numerous separate stocks are on the shelves of Iceland, the coast of Norway, the Barents Sea, the Faroe Islands, off western Scotland, the North Sea, the Irish Sea, the Celtic Sea and in the Baltic Sea.

The northeast Atlantic has the world's largest population of cod. By far, the largest part of this population is the northeast Arctic cod, as it is labelled by the ICES, or the Arcto-Norwegian cod stock, also referred to as skrei, a Norwegian name meaning something like "the wanderer", distinguishing it from coastal cod. The northeast Arctic cod is found in the Barents Sea area. This stock spawns in March and April along the Norwegian coast, about 40% around the Lofotenarchipelago. Newly hatched larvae drift northwards with the coastal current while feeding on larval copepods. By summer, the young cod reach the Barents Sea where they stay for the rest of their life, until their spawning migration. As the cod grow, they feed on krill and other small crustaceans and fish. Adult cod primarily feed on fish such as capelin and herring. The northeast Arctic cod also shows cannibalistic behaviour. Estimated stock size was 2.26 million tonnes in 2008.

The North Sea cod stock is primarily fished by European Union member states and Norway. In 1999, the catch was divided among Denmark (31%), Scotland (25%), the rest of the United Kingdom (12%), the Netherlands (10%), Belgium, Germany and Norway (17%). In the 1970s, the annual catch rose to between 200,000 and 300,000 tons. Due to concerns about overfishing, catch quotas were repeatedly reduced in the 1980s and 1990s. In 2003, ICES stated there is a high risk of stock collapse if then current exploitation levels continued, and recommended a moratorium on catching Atlantic cod in the North Sea during 2004. However, agriculture and fisheries ministers from the Council of the European Union endorsed the EU/Norway Agreement and set the total allowable catch (TAC) at 27,300 tons. Seafood sustainability guides, such as the Monterey Bay Aquarium'sSeafood Watch, often recommend environmentally conscious customers not purchase Atlantic cod.

The stock of northeast Arctic cod was more than a million tons following World War II, but declined to a historic minimum of 118,000 tons in 1987. The catch reached a historic maximum of 1,343,000 tons in 1956, and bottomed out at 212,000 tons in 1990. Since 2000, the spawning stock has increased quite quickly, helped by low fishing pressure. However, there are worries about a decreased age at first spawning (often an early sign of stock collapse), combined with the level of discards and unreported catches. The total catch in 2003 was 521,949 tons, the major fishers being Norway (191,976 tons) and Russia (182,160 tons).